Passively Q-switched diode-pumped solid state lasers have been widely used in applications requiring optical sources of pulsed radiation because they are efficient, compact and available at several wavelengths in visible and UV wavelength ranges. However, in their standard mode of operation they produce pulses at a fixed repetition frequency often referred to as a free-running frequency, which makes it difficult to adopt these lasers in applications requiring adjustable pulse repetition rate. Although methods of varying the pulse repetition rate of passively Q-switched lasers have been disclosed, difficulties remain in adopting these lasers in those applications requiring repetition-rate independent lasing characteristics, such as pulse energy, pulse to pulse stability, beam profile and divergence.
An example of such applications is matrix-assisted laser desorption/ionization (MALDI) mass spectroscopy, typically requiring UV lasers with pulse repetition rate in the range from tens of Hz to few kHz and very stable characteristics.
In passive Q-switching, a laser cavity typically includes a gain element, typically a small piece of laser crystal, also referred hereinafter as a laser microchip, and a saturable absorber sandwiched between two laser mirrors. The gain medium is pumped by a source of pump radiation, preferably a high-power laser diode. In a most common cw pumping mode, the laser diode continuously pumps the gain medium at a constant power level Pump; at a start of operation the saturable absorber is in a high-loss state preventing a lasing action by effectively blocking the lightpass within the cavity. The continuous pumping energizes the gain element by inducing a build-up of population inversion of a lasing transition in the gain medium above a threshold level. Once the population inversion under the cw pumping reaches a certain critical level, intensity of associated amplified spontaneous optical radiation at a wavelength λ of the laser transition reaches a level sufficient to cause an abrupt reduction of an optical loss in the saturable absorber, opening up the laser resonator and leading to a fast avalanche-like formation of a laser pulse. The laser pulse drains the resonator from the accumulated energy leaving the population inversion well below the threshold level. After the pulse ends, the saturable absorber switches back to the high-loss state, and the process of the population inversion build-up starts again. The time Bup required for the population inversion to reach the critical level from the low after-pulse level, commonly referred to a build-up time, determines the free-running pulse repetition rate Ffree≈1/Bup.
FIG. 1 illustrates the free-running operation of a passively Q-switched laser by means of a timing diagram showing the cw pump power Pump and the optical output of the laser in a form of optical pulses 20 separated by the time interval Bup.
FIG. 2 shows a timing diagram illustrating a prior-art method of operation of a passively Q-switched laser at a pulse repetition rate F other than Ffree by employing pulsed pumping. In this method, the pumping 11 is stopped after each pulse 21 and starts again after a selected time delay 13 so that a time period between pulses T≧Bup. The pulse repetition rate in this case F=1/T<Ffree.
A drawback of this mode of operation, hereinafter referred to as a fixed frequency mode, is that a thermal loading of the gain element changes whenever the repetition rate F is changed, as is explained hereinafter. The varying thermal loading changes temperature distribution within the gain element, thereby changing the thermal lens which is typically created in the laser microchip due to heat generated by pump absorption in the gain element, which significantly affects many characteristics of the laser radiation such as pulse energy, beam divergence, pulse-to-pulse stability, etc.
The thermal loading of the gain element, that can be defined as heat generated in the microchip due to pump absorption, has to be therefore maintained constant throughout laser operation even when the repetition rate is changed. This thermal loading is a function of an average pump power Pave dissipated in the gain element in the form of heat. For the aforedescribed pulsed method of operation, the average dissipated pump power Pheat is approximately given by equation (1):Pheat=αβPave   (1)
where α is a fractional thermal loading, i.e. a part of the absorbed optical power which is dissipated as heat within the gain element, and β is a fraction of the average optical power which is absorbed by the microchip.
For a diode-pumped laser, the average optical power in the fixed frequency mode is given by equation (2):Pave=(I−Ith)Rη  (2)
where η is a slope efficiency of the laser diode, which is typically around 1 W/A, Ith and I are respectively the threshold current of the laser diode and the laser current providing the pump pulse, and R is the diode duty cycle defined as a ratio of the build-up time to the period between pulses:R=Bup/T=F Bup.   (3)
It follows from equations (1), (2) and (3) that the heat dissipated within the microchip, or the thermal load, is directly proportional to the duty cycle R and changes proportionally to the repetition rate F.
J. Zayhosky et al. in an article published in IEEE J. of Quantum Electronics, vol. 38, n. 11, pp. 1449-1454, 2002, disclosed a q-switched laser system wherein the pump power Pump is so high that the build-up time Bup is small compared to the pulse period T in a range of repetition rates of interest, so that the thermal loading remains approximately constant within this range. This method however requires a very high pump power and limits the range of possible repetition rate change.
U.S. Pat. No. 6,038,240, in the names of Deutsch et al., discloses a method and solid-state laser system for generating laser pulses with a variable pulse repetition frequency and constant beam characteristics, wherein an actively Q-switched laser is pumped with a train of pump pulses which duty cycle R is kept constant when the repetition rate changes, and wherein the laser pulse is triggered externally by opening an active Q-switch at a fixed time delay relative to the beginning of each pump pulse. This method, although providing certain benefits for actively Q-switched laser systems, for passively Q-switched laser is limited to a range of repetition frequencies between Ffree and 0.5×Ffree, since a pump pulse cannot be longer than 2Bup without triggering a second laser pulse.
U.S. Pat. No. 6,418,154, in the names of Kneip et al., discloses a pulsed diode-pumped solid-state laser with active Q-switching, wherein a Q-switch and a diode-laser array are cooperatively controlled by a controller such that laser output-pulses produced in response to pump-light pulses have the same energy independent of the time-interval between laser output-pulses. In this invention, a laser pulse is actively externally triggered as soon at a pump-light pulse is terminated, and the diode-laser array is arranged to deliver sufficient additional pump-light to the gain-medium, between termination of each pump-light pulse and initiation of a subsequent pump-light pulse, such that gain in said gain-medium is the same at the initiation of each pump-light pulse independent of the time interval between said pump-light pulses. The additional pump-light between pump-pulses is intended to maintain gain at a minimum level so to compensate for an exponential decay in time of the population inversion after a termination of pumping, which causes initial conditions of the laser at a beginning of a pump pulse to be dependent on the time period between consecutive pulses, thereby leading to variations of the laser pulse energy when the pump pulse repetition rate is changed. This method of pumping the gain medium, although appearing to perform its intended function for an actively Q-switched laser, may lead to an average pump-light power which is repetition-rate dependent, and therefore, at least in the case of microchip lasers with passive Q-switching, may lead to variations of laser characteristics in dependence on the repetition rate.
An object of this invention is therefore to provide a Q-switched laser apparatus and a method of operation thereof for generating sequences of laser pulses wherein characteristics of the laser pulses is substantially independent on their repetition rate in a wide range thereof.